The present invention relates to a liquid development apparatus and a liquid development and transfer apparatus for electrostatic latent image, in which toner particles with electric charge are suspended in an insulating liquid and development is performed by applying an electric field between the electrostatic latent image and development electrodes.
Description is given on the liquid development method referring to FIG. 1. A developing solution 3 is filled between a conductive development roller 1 and a dielectric layer or a photosensitive layer 2 where a latent image 5 is formed. In the developing solution 3, charged toner particles 4 having opposite polarity to the latent image charge are suspended in an insulating liquid. By an electric field created by the latent image charge 5, the charged toner 4 is attracted and development is performed. In this case, by short-circuiting between the dielectric layer or the photosensitive layer 2 and the development roller 1 to bring them to the same potential, electric field strength E in the developing solution is increased to attain effective development. By shortening the distance L to the dielectric layer or the photosensitive layer 2, the electric field strength E can be increased. However, if the distance L is too short, the quantity of the developing solution 3 decreases, and this leads to insufficient development. Thus, the distance L must be set to an optimal length.
More concretely, the liquid development apparatus of FIG. 1 is divided into two types: the one having a dish development electrode shown in FIG. 2 and the one having a rotating roller type development electrode shown in FIG. 3.
In the apparatus shown in FIG. 2, a dish type development electrode 11 is arranged face-to-face to a cylindrical electrostatic latent image carrier 10, and a fluid developer 12 is placed between them. Voltage with the same polarity as the electrostatic latent image from a bias power source 13 is applied on the development electrode 11 to prevent development (fogging) on the portion where latent image is not formed.
In the apparatus shown in FIG. 3, an application roller 20 is arranged face-to-face to an electrostatic image carrier 10, and voltage with the same polarity as the electrostatic latent image from a bias power source 13 is applied on the application roller 20. A fluid developer is injected on the application roller from a developer supply unit 21 in the form of nozzles, and the developer is supplied to the electrostatic latent image carrier 10 by the application roller. The application roller 20 may be immersed in the fluid developer 23, which is filled in a container 22, so that the developer may be supplied by rotating the roller.
In the apparatus shown in FIG. 2, bias voltage is applied to prevent development on the portion where an electrostatic latent image is not formed, but toner is electrodeposited on the surface of the development electrode 11 by bias voltage, thereby reducing the electrode's effectiveness and gathering the toner at a lower position as shown by the reference numeral 14. The toner gathered on the development electrode side provides an inverse bias voltage for a certain period of time up to the next development starting after the completion of the present development and builds up toward the electrostatic latent image carrier 10. Normally, the toner can be removed by cleaner. However, if the toner is dried and solidified on the electrostatic latent image carrier 10, it is not very easy to clean up. The closer the development electrode 11 is placed to the electrostatic latent image carrier, the more development is promoted. On the other hand, if the quantity of the developing solution is decreased or if the electrode is too close to the carrier, the developing solution forms a meniscus between the development electrode and the electrostatic latent image carrier, and the discharge is hindered. If this is dried and solidified, it is not very easy to clean it up.
In the apparatus shown in FIG. 3, it is possible to mechanically remove the toner attached on the application roller 20 by a blade 24. However, if the development apparatus is arranged in a transverse direction to the electrostatic latent image carrier 10 or if there is not a very wide space, it is difficult to uniformly provide the developing solution on the roller surface and to evenly supply the developing solution to the surface of the electrostatic latent image.
Thus, in the liquid development apparatus, DC bias voltage with the same polarity as the electrostatic latent image is applied on the development electrode to prevent development (fogging) due to residual potential on the electrostatic latent image. However, on the portion where the electrostatic latent image is not formed, the developer is electrodeposited on the development electrode due to DC bias voltage. As the result, the electric field on that portion is weakened, and this causes difficulties such as stripes or blurs. For this reason, in case of the dish type development electrode, bias voltage with the opposite polarity to the electrostatic latent image is applied for a limited duration to clean up the electrodeposited developer, while, when the electrostatic latent image is continuously formed, bias voltage of the opposite polarity cannot be applied. In case of the rotating roller type development electrode, it is possible to mechanically remove the electrodeposited developer by a doctor blade, but it is not possible to have a long developing time as in the case of the dish type electrode.
FIG. 4 shows a liquid development and transfer apparatus, in which an application roller 20 with bias voltage applied on it is arranged face-to-face to an electrostatic latent image carrier 10, and a fluid developer is injected to the application roller by a developer supply unit 21 in form of nozzles, and the developer is supplied to the electrostatic latent image carrier 10 by the application roller. Or, the application roller 20 may be immersed in a fluid developer 23, which is filled in a container 22, and the developer may be supplied by rotating the roller. After developing in this way, a recording paper 26 is pressed on the surface of the electrostatic latent image carrier 10 by a transfer unit 25, and the developed electrostatic latent image is transferred to the recording paper 26.
When a wet type development unit for each color is arranged in order to obtain a full-color image using the liquid development and transfer apparatus as shown in FIG. 4 and multicolor superimposing development is performed on the surface of the electrostatic latent image carrier and it is transferred to the recording paper or to a transfer intermediate medium, developing solution for each color may be mixed in some cases. In case the electrostatic latent image carrier consists of a photosensitive member having a photoconductive layer, it is necessary to perform exposure for another color through the developer layer of the color which has been developed already. In such case, it is very difficult to carry out color superimposing development on the photoconductive layer because light absorption by the developer layer occurs.
FIG. 5 shows a conventional type multicolor liquid development apparatus using development rollers. Development units 30 for each of 4 colors of Y, M, C and K are arranged face-to-face to an electrostatic latent image carrier 10, and these units independently move up and down with respect to the electrostatic latent image carrier 10 as shown by arrows B. Thus, multicolor superimposing development is performed by moving the development unit 30 up and down for each color. Each of the development units 30 is provided with one or more application rollers 30a. Squeeze rollers 30b are provided to make pairs with the application rollers, and solvent in excess for each color is recovered. It is needless to say that bias voltage with the same polarity as the latent image formed on the surface of the electrostatic latent image carrier 10 is applied on the application roller 30a as in the case of FIG. 4, and bias voltage is also applied on the squeeze roller 30b to scrape off the toner, which has not been used for development.
However, in the multicolor development apparatus of FIG. 5, the development rollers and the squeeze roller are arranged in pairs. Thus, as many squeeze rollers as the number of colors are needed. This requires the apparatus of large size, making it difficult to install in small space. Also, it is necessary to keep a constant distance between the squeeze rollers and the electrostatic latent image carrier, but it is difficult to keep constant distance because the squeeze rollers are moved together with the ascending or descending development rollers.